Piezoelectric crystal oscillator



July 13, 1954 w. s. MORTLEY PIEZOELECTRIC CRYSTAL OSCILLATOR Filed March16, 1950 2 Shee tsSheet l K2 QUQRTER WAVE LENGTH UNE 11'] QUARTER WAVE 1LEA/6TH LINE 4 Jufiy 13, 1954 w, s MQRTLEY 2,683,810

PIEZOELECTRIC CRYSTAL OSCILLATOR Filed March 16, 1950 2 Sheets-Sheet 2L1 6/ a1 a 7 w kmwh A 37 9 MW Patented July 13, 1954 2,683,810PIEZOELECTRIC CRYSTAL OSCILLATOR Wilfrid Sinden Mortley, Great Baddow,Chelmsford, England, assigncr to Marconis Wireless Telegraph CompanyLimited, London, England,

a British company Application March 16, 1950, Serial No. 149,978

Claims priority, application Great Britain March 30, 1949 6 Claims.

This invention relates to piezo electric crystal oscillators and moreparticularly to oscillators of the customary kind in which a piezoelectric crystal is maintained in oscillation by a valve.

As is well known, where it is required to obtain the highest possiblefrequency stability from a crystal oscillator, it is best to cause thecrystal to vibrate at a frequency as near as possible to that of seriesresonance. It is also a requirement that a suitable impedance should bepresented to the maintaining valve and that the Q value of the crystalshould not be appreciably reduced by the circuits coupling it to thesaid valve.

The present invention seeks to satisfy these requirements in a simpleway.

According to this invention a piezo electric crystal is connected as acoupling link between two quarter wave lines the outer end of one ofwhich is coupled between the grid point and cathode point of amaintaining valve and the outer end of the other of which is coupledbetween the anode point and the cathode point of said maintaining valve.

The quarter wave lines may be of any type, real or artificial, i. e.with disturbed impedances or composed of lumped impedances. In the caseof artificial quarter wave lines they may be of the series inductanceand shunt capacity type or vice versa or they may be of the mixed typeprovided that at the working frequency both series arms are capacitativeor both are inductive and at this frequency w2L1C1=1 and w2L2C2=1 wherew is the frequency in angular measure, L1 and C1 are respectively theinductance and capacity values of one quarter wave line, and C2 and L2are respectively the inductance and capacity values of the other quarterwave line.

The two quarter wave lines need not be of the same characteristicimpedance and indeed there may be some advantage (from the point of Viewof reducing instabilities due to valve capacity) by making the lineadjacent the grid of the valve of lower impedance than the other. On theother hand, for example, in the case of a circuit of medium-highstability without automatic gain control, it may be of advantage tomaking the line adjacent the grid of the valve of higher impedance thanthe other, for then the crystal oscillation amplitude, and therefore itsself-heating, would be less.

The conditions for oscillation are satisfied by the equation Z1Z2=T/gmwhere Z1= /L1/C1=the characteristic impedance of one line Z2=/L2/C'2=the characteristic impedance of the other line r=seriesresistance of the crystal gm=the working value of the mutual conductanceof the valve.

The invention is illustrated in the accompanying drawings in whichFigure 1 is a general diagram of an embodiment of the invention, whileigs. 2 to 5 show various forms of lumped impedance line which may beemployed for the lines represented by rectangles in Fig. 1. In all thefigures line terminals are numbered, the same numerals being used forthe same terminals throughout.

Referring to Fig. 1, the crystal X and two associated quarter wave linesKI and K2 constitute a four terminal network of which the terminals 'l,8 of one end pair are connected respectively to the control grid G andcathode C of a maintaining valve V (which may be of any type but whichfor the sake of simplicity will be assumed to be a triode) while theterminals I, 2 of the other end pair are connected between said cathodeC and the anode A of the maintaining valve. One terminal of eachpair-the terminals I and il-is common and the cathode C is connected tothis common point. The crystal X is in series between the line terminals3 and 6 and the terminals 4 and 5 are connected together. A resistance Rmay be connected across terminals 5 and B or across terminals 3 and 4(or resistances may be connected in both these places) to preventoscillation in unwanted modes.

Figs. 2 and 3 show two of the many forms which may be adopted for thequarter wave lines K1 and K2 of Fig. 1. Referring to Fig. 2 and startingwith the anode terminal 2 of the network the series arm between thisterminal and the grid terminal 1 consists of an inductance of value L1in series with the crystal X which is in turn in series with aninductance of value L2. There are four shunt capacities one of value C1between the anode and cathode terminals I, 2 one of the U same valuebetween the terminals 3, 4 (the remaining end of L1 and the commonterminal), one of value C2 between the terminals 6, 5 (the crystal endof L2 and the said common terminal), and one of the same value C2 acrossthe remaining pair of terminals 1, 8.

In the form shown in Fig. 3 the series inductance of values L1 and L2 ofFig. 2 are replaced by series condensers of values Cl and C2respectively and the shunt: condensers of values 01 and C2 of Fig. 2 arereplaced by shunt inductances of values L1 and L2 respectively.

Figs. 4 and 5 are, respectively, the T section equivalents of the Pisections. of Figs. 2 and 3. It is thought that, in view of thedescription already given of Figs. 2 and 3, Figs. 4 and 5 will belargely self-explanatory since throughout Figs. 2 to 5 the variouscircuit elements are indicated by their values.

The invention is not limited to the use of lines with elementsdimensioned precisely as above described and in particular if, as willusually be the case, Z1 and Z2 (the characteristic impedances of K1 andK2 respectively) are both large with respect to r (the resistance of thecrystal) the shunt. elements adjacent the crystal are not critical invalue and may often be omitted altogether without serious loss ofstability. If desired resistances may be connected across these elements(in Fig. 1 such a resistance is across terminals 6, 5) or across thecrystal in order to suppress any unwanted mode of oscillation caused bycoupling through the shunt capacity of the crystal.

For true series resonant operation the crystal shunt capacity should bebalanced by a suitable shunt inductance and such an inductance may beprovided if required. In practice, however, the frequency departure fromthat of true series resonance caused by leaving the shunt capacity ofthe crystal unbalanced is negligible.

Small adjustments of frequency may be made by providing shunt reactancesat either or both ends-of the network, i. e. between grid and cathodeand/or between anode and cathode.

We claim:

1. A piezo-electric crystal oscillator arrangement comprising apiezo-electric crystal, a valve having at least a cathode, a controlgrid and an anode, a four terminal quarter wave line having the twoterminals at one end connected between the control grid and the cathodeof said valve, a second four terminal quarter wave line having the twoterminals at one end connected between the anode and the cathode, ofsaid valve, and a coupling link including said crystal connected betweenthe remaining pairs of terminals at the other end of said quarter wavelines.

2.. A piezo-electric crystal oscillator arrangement as set forth inclaim 1 wherein the two quarter wave lines have distributed constants.

3. A piezo-electric crystal oscillator arrangement as set forth in claim1 wherein the two quarter wave lines are constituted by lumpedconstants.

4. A piezo-electric crystal oscillator arrangement as set forth in claim1 wherein the two quarter wave lines are of the same characteristicimpedance.

5. A piezo-electric crystal oscillator arrangement. as set forth in.claim 1 wherein the two quarter wave lines are of differentcharacteristic impedances.

6. A piezo-electric crystal oscillator arrangement as set forth in,claim 1 which includes a resistance connected across at least one of thequarter wave lines.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,053,524 Heegner Sept. 8, 1936 2,165,517 Stevenson July 11,1939 2,259,528 Mason Oct. 21, 1941 2,345,491 Mason Mar. 28, 19442,551,809 Mortley May 8, 1951

